The project is designed to test the hypothesis that the known deficiency of a Beta-adrenergic hypersensitivity in pineal gland to light stimuli in aged animals begins early in the aging process in females and is correlated with or causally related to changes in reproductive cycles and fertility. The study will focus on middle aged rats and will compare and contrast the ability of subgroups of these animals with young and aged rats with regard to pineal Beta-adrenergic receptor density changes induced by light, changes in melatonin levels in serum and pineal tissue and the effects thereon of high versus low estrogen levels in ovariectomized animals. Of special interest will be the temporal relationship between altered Beta-receptor activity, melatonin production and age of onset of irregularities in estrus cycles.